Bodging the Brakes Safely
I recently bought a second
(or third or fourth or fifth) hand Linear recumbent
bicycle.
It’s an interesting design,
consisting of a single extruded aluminium alloy beam with all the elements –
seat, steering, drive and wheels hung from it. Mine dates from the mid 1980s or
early 1990s. This is suggested by the fact that it was fitted with a triple
front chainring (probably not original), anodised chainstay and it still has
caliper brakes, though I’m not sure if they’re the originals. And it’s the
caliper brakes that are a safety and performance issue for me. You see, someone
upgraded the shifters at some point. They used lovely Shimano click-shift
controls – the thumb and index finger ones - with integrated brake levers. And,
of course, these levers are designed for linear pull cantilever (or ‘Vee’)
brakes. So what? Well, linear pull cantilevers need to travel twice as far as
caliper brakes. So the levers pull twice the length of cable, but only transmit
half the force to the caliper. So, when you try to stop a bike with a ‘Vee’
brake lever on a caliper brake what happens…?
Not… a… lot…!
So I needed to reduce the
amount of cable pulled and increase the force pulling it. Remember school days
- physics class lessons on levers? To get twice the travel you move the lever
twice the distance from the pivot. Yes? That’s how vee brake levers work. Trouble
is, that halves the force when it doubles the travel, doesn’t it. That’s what’s
happened with the new levers. So, how do I reduce the travel and increase the
force in this situation? Given the opposite scenario – vee brake with caliper
lever, there’s a device to fix the problem. It’s called a travelator or some
such, and apparently consists of an eccentric pulley to increase the cable
travel to vee brake requirements. But it fixes to the brake lever, I believe.
(I haven’t actually studied one.) So it isn’t ‘reversible’ for the opposite
scenario.
My solution isn’t so high
tech, but it does spring directly from those physics lessons. Remember basic
pulleys. You can calculate the mechanical advantage of a pulley system by
counting the number of lines on the tension side of a pulley system. And I only
want to double the force and halve the distance, so it’s the simplest of pulley
systems. A fixed end to the cable pulled by the lever passes over one pulley.
This gives two tension lines and a mechanical advantage of two to one (2:1).
And it halves the movement of the output cable which is fixed to the pulley
block. Exactly what’s needed.
Here’s the finished
prototype. The lever pulls on the upper cable on the right. The pulley travels
to the right, drawing the attached assembly and cable with it. The caliper
brake is on the left, actuated by the left hand cable.
the practicalities weren’t
too complicated. i found a suitable pulley on a lady’s frame. it had been used
to direct the brake cable upward to the caliper after it had followed the
sloping top tube down to the base of the seat tube (a 1:1 scenario).
the pulley block i
fabricated from tubing which i cut from another scrap bike and flattened,
shaped and drilled for the pulley screw and bushing and the barrel end of the
output cable.
The tension side cable stop
was fabricated from more of the same material.
It works beautifully on the
bike. The brake operation is progressive and quite positive considering the
amount of slack the system. I’ll eventually dismantle it all, wire brush it and
paint it with black hammerite so it ‘disappears’ into the side of the bike, and
grind a bit off the back side stop nut so it doesn’t touch the frame. But for
now I’m thinking how to squeeze the second one in behind this one so I can have
a powerful rear brake on the bike too! I think the pulley assembly and cable
stop could come forward. This would probably leave enough space for another.
This modification is probably not so easy to do on a bike with a round tube
frame, but for the Linear it’s a breeze.
-- Denis Buckley